1. Field of the Invention
The present invention relates to catalyst components for olefin polymerization which can produce polyolefins, and in particular polypropylene having high crystallinity and relationship high melt flow rates (MFR), and consequently, suitable processing properties.
2. Description of the Prior Art
The homopolymers and copolymers of propylene generally have certain properties that are unsatisfactory for specific applications. It therefore becomes necessary to modify certain characteristics during the manufacture of the polypropylene to render the polymer more useful for a certain end results. For example, if the rigidity of the polymer or copolymer of propylene is improved, it is possible to reduce the thickness of the resulting molded product formed from it.
There are numerous polymerization and copolymerization processes and catalyst systems in the prior art from which it is possible to tailor a processing catalyst system to obtain a specific set of properties of a resulting polymer or copolymer. For example, in certain applications, a product with higher melt flow rate is desirable. Such a product has a lower melt viscosity than a product with a lower melt flow rate. Many polymer or copolymer fabrication processes which operate with high shear rates, such as injection molding, oriented film and thermobinded fibers, would benefit from a lower viscosity product by improving through-put rates and reducing energy costs. Generally, olefin polymers obtained by using an active catalyst component of the magnesium (MgCl2) supported type have a limited melt flow rate range and mechanical properties. As indicated, however, for certain applications, polypropylene polymers which flow readily during melting have improved processing, characteristics.
A discovery of more appropriate co-catalysts or electron donors to accompany supported magnesium catalyst components have been of great benefit to improving the efficiency of the catalyst system and the quality control of the polymer product. In such catalyst systems, the cocatalyst activates the catalyst and provides initiation of a polymer chain. The cocatalyst that has historically worked well with magnesium supported catalysts is organo aluminum compounds, most typically triethylaluminum (xe2x80x9cTEALxe2x80x9d), or other trialkyl aluminum compounds. Examples of other useful organoaluminum compounds include an alkylaluminum dihalide, a trialkoxyaluminum, a dialkylaluminum halide and a triisobutyl aluminum.
An electron donor compound is used in the polymerization reactor to control the stereoregularity and form of the polymer. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or groups of compounds with which it is especially compatible. Discovery of an appropriate type of electron donor can lead to significant improvement in the properties of the polymer product such as molecular weight distribution and melt flow. Discovery of a specific group of electron donors for magnesium supported catalysts that would provide beneficial results would be highly advantageous.
The present invention is directed to the use of a mixture of silane electron donors. In Mitsui EP 385765A (published Sep. 5, 1990), there is demonstrated a catalyst system composed of a magnesium supported titanium catalyst component in combination with two silane electron donors, a specific donor mixture embodiment of which is directed to dicyclopentyl dimethoxysilane (xe2x80x9cDCPMSxe2x80x9d) and propyltriethoxysilane (xe2x80x9cPTESxe2x80x9d). In U.S. Pat. No. 5,100,981, disclosed is a catalyst system composed of a magnesium supported titanium catalyst and a mixture of two electron donors, cyclohexylmethyl dimethoxysilane (xe2x80x9cCMMSxe2x80x9d) and phenyltriethoxy silane (xe2x80x9cPESxe2x80x9d).
It has now been surprisingly found that the use of two different organosilicon compounds as electron donors in combination with a magnesium supported catalyst is capable of generating highly isotactic polypropylene polymers having a moderately broad molecular weight distribution as well as relatively high melt flow rates.
It has been found that a catalyst system based on a solid magnesium supported catalyst component, an organoaluminum cocatalyst component, and a mixture of two different electron donors, one of which functionally dominates the other, is capable of achieving highly crystalline polyolefins having the characteristics of moderately broad molecular weight distribution and relatively high melt flow rates. In a preferred embodiment, two organosilicon compound electron donors, dicyclopentydimethoxysiliane (xe2x80x9cDCPMSxe2x80x9d) and tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d) have been found to provide an optimum electron donor mixture in the instant catalyst system invention. It has been discovered that DCPMS acts as the dominant donor in combination with the weaker donor TEOS. It is found that a mixture of the two donors in a catalyst system generates isotactic polypropylene having product characteristics similar to the polypropylene product generated by a supported catalyst system employing 100% DCPMS such as high crystallinity and low melt flow rate. This discovery enables the use of the weaker donor to make polyolefin polymers in a first stage reaction with the polymer characteristics of the TEOS donor, i.e. high MFR, and polymerizing the resulting polyolefin in a second stage reaction with a mixture of the weak donor (xe2x80x9cTEOSxe2x80x9d) and the stronger donor (xe2x80x9cDCPMSxe2x80x9d) to achieve a final highly crystallinity polyolefin polymer having beneficial characteristics of relatively high MFR and moderately broad MWD, these polymer characteristics being otherwise independently obtainable by the sole use of each silane electron donor. In summary, the use of the present dual donor system enables the production of a final polyolefin product with relatively high MFR, a moderately broad MWD similar to that of DCPMS generated polymers.
The surprising and unexpected result in the use of the dual donor system of the present invention lies in the dominant feature of the DCPMS. Ordinarily, it would have been expected that the use of DCPMS in combination with TEOS would result in physical characteristics in the ultimate polymer attributable to the proportionate amount of each electron donor used. However, the discovery that the use of these two donors in a catalyst system generates polyolefins with properties attributable to the dominant electron donor, DCPMS, has resulted in the development of a two stage polymerization process in which a high MFR polyolefin product is made in the first stage through the use of the TEOS donor, while the second stage reaction utilizing a mixture of TEOS/DCPMS enables production of a highly crystalline polyolefin having the characteristics of moderately broad molecular weight distribution and relatively high MFR. In summary, the use of TEOS as an electron donor in a first stage olefin polymerization reaction, followed by introduction of the electron donor DCPMS in a second stage reaction has resulted in the production of crystalline polyolefin products having the characteristics of moderately broad polymer MWDs and relatively high MFRs.
The present invention provides a catalyst system for the polymerization of olefins comprising:
(A) a solid magnesium supported titanium catalyst component;
(B) an organoaluminum cocatalyst; and
(C) a Dual Donor catalyst system comprising (1) a first electron donor compound, and (2) a second dominating electron donor compound, wherein, the MFR (a) of a polyolefin obtained by polymerizing an alpha-olefin in the presence of dominating electron donor (2) and, the MFR (b) of polyolefin polymerized under the same polymerization conditions, but using a mixed donor system of 50 mole percent (1) and 50 mole percent (2), satisfy the relationship.       LOG    ⁢          xe2x80x83        ⁢          "LeftBracketingBar"                        [                      MFR            ⁢                          xe2x80x83                        ⁢                          (              a              )                                ]                          [                      MFR            ⁢                          xe2x80x83                        ⁢                          (              b              )                                ]                    "RightBracketingBar"        ⁢          xe2x80x83        ⁢    is    ≤      1.2    .  
The present invention is further directed to a process for making a homopolymer or copolymer, or a reactor blend of homopolymer and copolymer (impact copolymer), of an alpha-olefin having a relatively high melt flow rate, high crystallinity and moderately broad molecular weight distribution comprising polymerizing an alpha-olefin in a multi-stage reaction in the presence of (1) a high activity magnesium supported titanium catalyst system, (2) an organoaluminum cocatalyst, and (3) an electron donor mixture of two different compounds in at least two stages, comprising:
(A) in the first stage, polymerizing the alpha-olefin in the presence of a first electron donor (i) to produce a polyolefin, and
(B) in another stage, further polymerizing the polyolefin polymer of stage one in the presence of a donor mixture of the electron donor (i) and a dominating electron donor (ii) to achieve a final polyolefin.